1. Field of the Invention
The present invention relates generally to an anti-icing management system for a gas turbine engine and, more particularly, to such a system which utilizes a uniquely operated solenoid valve to provide on an as required basis, only the necessary amount of heat to prevent the formation of ice at the engine inlet.
2. Description of the Prior Art
It is common in modern gas turbine powered aircraft to find systems which perform a thermal anti-icing function utilizing hot, compressed gas from the discharge region of the engine compressor called "bleed air." In such aircraft, a portion of the total bleed air available is routed to the various components in the aircraft which are subject to icing and then exhausted overboard.
For purposes of describing the present invention, it will be understood that the term "anti-icing" refers to the prevention of the formation of ice in the first place whereas the term "de-icing" refers to the reduction, or elimination, of ice after it has begun to form. However, it will further be understood that although the term "anti-icing" is consistently used throughout the disclosure, the invention is not to be so limited, but is applicable to de-icing systems as well.
Historically, helicopters have had limited operational capability in icing environments due to their susceptibility to rotor blade icing. Military strategists present numerous scenarios of possible future conflicts involving helicopters in temperate and arctic regions. In these instances, engagement would likely occur in poor weather conditions that are conducive to icing environments, and helicopter mission performance under such conditions could be decisive. Recognizing this fact, the next-generation of smaller helicopters will require all-weather capability and will be equipped with rotor blade deicing systems. As power requirements for rotor blade de-icing are large, research has been directed towards minimizing these requirements using an efficient management control system. However, the need for similar engine anti-icing management systems has been largely overlooked.
High efficiency engines are generally more sensitive to off design operating conditions produced by compressor bleed air anti-icing systems and, without careful bleed air management, can severely penalize engine performance. For example, an advanced technology helicopter turboshaft engine, equipped with an inlet particle separator, requiring 2.5 percent compressor bleed for the critical anti-icing design point, will realize engine performance penalties of 4 to 5 percent SFC (specific fuel consumption) increase, 7.5 to 10 percent power loss, and 40.degree. C. increase in turbine gas temperature. These values exceed the power and SFC requirements established by military specifications for anti-icing operation. The cumulative operational effect is a reduction in mission capability and engine life.
Conventional engine anti-icing systems are very wasteful of compressor air bleed heating because operation is set for the most severe anti-icing design point condition even when a less severe or no icing condition is encountered. Preliminary analysis on the accumulative effect of such factors as the probability of encountering icing in clouds, and operating at higher ambient temperatures and engine power conditions than the critical anti-icing design point (e.g. typically -20 .degree.C. and flight idle conditions) indicate that only 5 percent of the total bleed energy is being effectively used. Stated another way, when using a conventional engine bleed anti-icing system, 95 percent of the bleed energy can be wasted. Thus, the engine must work harder at higher turbine temperatures resulting in decreased engine life.
To achieve an energy efficient anti-icing bleed management system, it is only necessary to produce sufficient heating to maintain critical inlet surface temperatures above the water freezing temperature when encountering meteorological icing conditions.
Current ice protection systems, particularly anti-icing systems for helicopter engines, however, are not managed for efficient optimization and as a result can adversely impact mission performance and operation. Such systems for helicopter engines, e.g., T53, T55,T58, T64, LT10l, and the like, use a compressor hot air bleed source to heat critical engine inlet surfaces. The pilot actuates a simple on/off bleed valve when encountering potential icing conditions, i.e., typically at ambient temperatures below 5.degree. C. in the presence of visible moisture (i.e., a cloud). The bleed valve is designed for a fail safe `on` condition. This system provides a convenient, dependable heating source in a compact package. However, it is exceedingly energy inefficient.
This inefficiency is due to a number of factors. For example, anti-icing bleed may be actuated by the pilot even when no icing is encountered but merely because the presence of clouds make it appear as being likely to the pilot. Also, excessive bleed energy is consumed when operating above a critical meteorological design anti-icing point, e.g. ambient temperatue, T.sub.o equal to -20.degree. C. Still another factor causing the inefficiency is that excessive bleed energy is consumed when operating the engine above a critical anti-icing design point chosen for that particular engine, generally during idle operation. When using a conventional engine bleed anti-icing system, approximately 95 percent of the bleed energy is typically wasted.
Typical of the prior art relating generally to the general field of the present invention is the U.S. Pat. No. 2,868,483 to Krueger which discloses the use of a closed-loop anti-icing system which utilizes heated air extracted from the compressor manifolds of a gas turbine engine. Both the temperature and pressure of air flowing through the wing ducts are monitored. In the event prestablished limits are exceeded, a dump vent may be actuated or the flow of heated air restricted, but in neither instance with any concern for economy of the operation.
More closely allied to the present invention, however, are the U.S. Pat. Nos. 3,057,154 to Sherlaw et al, 3,258,229 to Larson, 3,693,916 to Tritt et al and 3,891,164 to Hertrick et al. Each of these patents discloses a valve for use in an anti-icing system which is positioned in an air passageway. In each instance the valve can be selectively opened and closed for control ling passage of the fluid therethrough. The latter three patents, in fact, disclose the use of solenoid operated valves.
However, none of the patents is concerned in any manner with economy of operation, that is, with admitting only a minimally sufficient flow of heated air to perform the anti-icing function based on an independent source of pertinent information.